Sustaining a uniform thin liquid film of liquids with moderate surface tension over area has remained a challenge for a long time, particularly because of film rupture caused by various dewetting forces such as body forces and/or a combination of dispersion forces and thermal fluctuations. Falling film evaporators produce liquid films but the film thickness varies along the direction of flow.
Evaporation heat transfer techniques are attractive because of the increases in heat transport that can be realized by means of latent heat exchange. Heat pipes, cold plates, vapor chambers and thermosyphons are a few examples of such devices. The efficiency of heat spreading in heat pipes and vapor chambers sometimes relies on the wick structure, the details of which determine the efficiency of heat removal (evaporation) as well as the heat flux that can be handled (passive pumping). Some heat pipe designs are still plagued with large evaporator resistances, primarily because the wicks have not been optimized to the extent possible. Wick structures include sintered wicks, microgrooves, meshes and felts. In some wick structures, an evaporating meniscus forms the basic operational unit for heat transport.
Thin-film evaporation, which is the evaporation occurring near a solid-liquid-vapor junction, has been claimed to be the most efficient mode of heat transfer in such devices, delivering heat transfer coefficients of about 106 W/m2K. Copper-water and copper-methanol are some of the commonly used wick-liquid combinations. Because of the inherent nature of these wick structures, liquid tends to form a meniscus of spatially varying thickness, which leads to inefficient heat transport; the high contact angles of the wick-liquid combination also contribute to this behavior.
What is needed are methods and apparatus for improving the efficiency of evaporative cooling. Various embodiments of the present invention do this in novel and unobvious ways.